Alloys

An alloy is a mixture of two or more elements, where at least one element is a metal. Many alloys are mixtures of two or more metals.

Comparing properties of alloys and pure metals

Many pure metals are too soft for many uses. They can be made harder by adding another element to the pure metal, so forming an alloy. This explains why an alloy often has more uses than the pure elements it is made from.

Pure iron, for example, is very soft. Adding a small amount of tungsten to iron makes tool steel, which is harder than pure iron. Steels are examples of alloys. There are many types of steel.

SteelElementsProperties
Mild steelCarbon and ironEasy to bend and pull into wires
Tool steelTungsten and ironHard, can be heated to high temperatures
Stainless steelChromium and ironHard, does not rust easily

Explaining alloy hardness

In the solid state, a pure metal has a giant metallic structure. The atoms are arranged in layers. When a force is applied, the layers may slide over each other. The greater the force needed, the harder and stronger the metal.

In a pure metal, the force needed to make the layers slide over each other is small. This explains why many pure metals are soft.

In an alloy, there are atoms of different sizes. The smaller or bigger atoms distort the layers of atoms in the pure metal. This means that a greater force is required for the layers to slide over each other. The alloy is harder and stronger than the pure metal.

Atoms of two different sizes packed together to form an irregular arrangement.The metal lattice structure is distorted in alloys
Question

Explain why steel, which is an alloy of iron, is harder than pure iron.

Steel contains atoms of other elements as well as iron. These atoms have different sizes to iron atoms, so they distort the layers of atoms in the pure iron. This means that a greater force is required for the layers to slide over each other in steel, so steel is harder than pure iron.

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